The sensitivity of human auditory sense depends on the frequency. For example, low-frequency components and high-frequency components are less audible when the sound volume is small. Characteristics of this kind are described as an equal loudness curve in ISO226: 2003.
In view of this, in conventional audio signal processing devices, the frequency characteristics are corrected according to the equal loudness curve by performing processing of increasing the levels of low-frequency components and high-frequency components as the sound volume setting value of a master volume decreases (refer to Patent document 1, for example).